Project Summary (Public Abstract) When prescribing a prosthetic foot, clinicians face a dizzying array of choices as more than 200 different prosthetic feet are available. While these conventional prosthetic feet primarily function in the sagittal plane, the intact foot and ankle comprise a complex set of joints that allow rotation in multiple planes of motion. Some of these motions are coupled, meaning rotation in one plane induces motion in another. One such coupling is between the sagittal and transverse planes. For every step, plantar- and dorsi-flexion motion is coupled with external and internal rotation of the shank relative to the foot, respectively. There is no prosthetic foot available for prescription that mimics this natural coupling. The purpose of the proposed research is to develop a passive prosthesis (Pivot-Flex Foot) that mimics the natural coupling of the intact limb and determine if this coupling can reduce transverse-plane socket torque and compensatory gait biomechanics when compared to a standard-of-care prosthesis. We anticipate the long-term outcomes from wearing a Pivot-Flex Foot are a lower incidence of soft tissue and compensatory gait injuries. To investigate the need for this coupling, we have already built a torsionally adaptive prosthesis (TAP) where the coupling ratio between the transverse- and sagittal-planes can be independently controlled with a motor. The TAP can be used to discover the optimal coupling ratio, but is a bit unwieldy for long-term, everyday use. A more robust solution for daily wear is the Pivot-Flex Foot, a passive prosthesis we have built using industry standards. We plan to use the TAP to identify the optimal coupling ratio and then build a selection of Pivot-Flex Feet for testing by individuals with lower limb amputation. Our proposed research has two specific aims: (1) To identify the optimal coupling ratio between transverse- and sagittal-plane motions using a novel, torsionally adaptive prosthesis. We propose to conduct a human subject experiment (n=15) with below-knee amputees wearing the motor-driven and computer controlled TAP. Participants will walk in a straight line and in both directions around a circle while we vary the coupling ratio between transverse- and sagittal-plane motions. Participants will be blinded to the coupling ratio. We hypothesize that: (1) a coupling ratio exists that minimizes undesirable transverse-plane socket torque and (2) there will be a coupling ratio that amputees prefer. The results from this test will be used to specify the coupling ratio of the more practical Pivot- Flex Feet we plan to test in specific aim 2. (2) To determine if a passive prosthesis with an optimal coupling ratio (Pivot-Flex Foot) can reduce transverse-plane socket torque and compensatory gait biomechanics when compared to a standard-of-care prosthesis. We propose to design and build Pivot-Flex Feet of various sizes and stiffnesses that can be worn by a majority of below-knee amputees. In a blinded crossover experiment, participants (n=15) will be fit with the Pivot-Flex Foot and the ssur Vari-Flex XC Rotate prostheses (random order), and then wear each for two weeks before we test them. The tests will involve walking in a straight line and in both directions around a circle while we measure their gait biomechanics. We hypothesize that the Pivot- Flex Foot will reduce transverse-plane socket torque and the work performed by the hip when compared to a standard-of-care prosthesis. After the proposed project, we will have evidence regarding the need for providing coupled motion in a lower limb prosthesis. If the Pivot-Flex Foot proves efficacious in this laboratory-based study, we will investigate long- term outcomes, namely reduced incidence of soft tissue and compensatory gait injuries, and pursue commercial development.